1. Technical Field
This disclosure relates generally to gas turbine engines and, more particularly, to rotor blades for gas turbine engines.
2. Background Information
Typically, a rotor blade for a gas turbine engine includes an attachment (also referred to as an “attachment region”) and an airfoil. The airfoil extends between the attachment and a tip and has a concaved pressure side surface, a convex suction side surface, a leading edge and a trailing edge. The airfoil is sized such that when it is configured within the engine, a clearance gap is defined between the blade tip and the surrounding static structure (outer flowpath).
During operation, a stagnation point is formed near the leading edge of the airfoil. A stagnation point may be defined as a point in a flow field where velocity of the airflow is approximately zero. At the stagnation point, the airflow separates into a pressure side airflow and a suction side airflow. The pressure side airflow travels from the stagnation point to the trailing edge. The suction side airflow is accelerated around the leading edge and a portion of the suction side surface until it reaches a point of maximum velocity. Typically, the point of maximum velocity corresponds to a point on the suction side surface where the surface becomes relatively flat as compared to a relatively curved portion of the airfoil proximate the leading edge. Thereafter, the suction side airflow decelerates as it travels from the point of maximum velocity to the trailing edge of the airfoil.
Near the tip of the airfoil, a portion of the pressure side airflow (i.e., a leakage airflow) migrates through the tip clearance gap to the suction side airflow. This leakage airflow mixes with the suction side airflow forming a vortex. The vortex mixes out and disperses, causing relatively significant flow disturbances along the majority of the suction side surface. As a collective result of these flow disturbances, the efficiency of the engine is reduced.
Several approaches have been adopted to try to reduce the detrimental effects associated with leakage airflows. In one approach, the clearance gap is decreased by reducing tolerances between the tip of each rotor blade and the outer flowpath. This approach has met with limited success because the tolerances must still account for thermal and centrifugal expansion of materials to prevent interference. In another approach, a shroud is attached to the tips of the rotor blades. Although air may still leak between the shroud and the outer, static flowpath, the vortex induced losses are reduced. A downside to this approach is that a shroud typically adds a significant amount of mass to the rotor, which may limit rotor operational speeds and temperatures.